Tape drives store date onto tapes. Typically tape drives store multiple tracks of data across the width of the tape simultaneously with multiple read/write elements on the head. The spacing between the read/write elements on the tape heads is closely controlled. When the width of the tape changes, the spacing of the data tracks across the width of the tape also change. When the spacing between data tracks change, the data tracks on the tape may no longer be aligned to the read/write elements on the tape head. If the position of the tape head is adjusted to align to one track of data on the tape, other tracks may no longer be aligned and may not be able to be read simultaneously.
The width of the tape may change due to a number of different factors. Changes in temperature cause changes in the width and in the length of the tape. Increases in temperature cause increases in both the width and length of the tape. Changes in humidity also cause changes in the length and width of the tape, with an increase in humidity acting to increase both the width and length of the tape. Changes in tension also affect the width and length of the tape. Increases in tension cause the tape to lengthen, but cause the width to decrease.
Tape drives are typically used in controlled environments (i.e. temperature and humidity control). Tapes are generally removable from the tape drive and therefore may be stored outside of a controlled environment or transported outside of a controlled environment. For example, a tape may be shipped to a data center and put into the tape drive as soon as the tape arrives. If the data center is located in a northern area during the winter, the tape may be cold and dry when it is inserted into the tape drive. If the data center is located in a southern area during the summer, the tape may be hot and wet when it is inserted into the tape drive. The difference between a cold tape and a hot tape may be 100 degrees F. or more. The difference between a dry tape and a wet tape may be as much as 80% relative humidity.